1. Field of the Invention
The present invention relates to compounds containing cyclic amidines and other blocking agents, to a process for their preparation, and to their use in one-component adhesive systems.
2. Discussion of the Background
The great reactivity of the epoxide group in epoxy resins allows the epoxide group to be cured with "stoichiometric" amounts of a coreactant or with catalysts. When curing with "stoichiometric" amounts, the resin and curing agent are linked to one another by way of a chemical bond.
Direct linking of the epoxy resin molecules with one another is a feature of catalytic curing, giving rise to crosslinking of the epoxy resin molecules by way of ether groups. It is initiated by means of polymerization catalysts, such as strong bases, tertiary amines, metal salts or Friedel-Crafts catalysts. For example, DE-A 27 31 335 describes a method of curing 1,2-epoxide compounds with cyclic amidines. A further hardener system for epoxy resins, which is used in about 50% of all heat-curing epoxy systems, is accelerated dicyandiamide. Based on this, one-component ("one-pack") adhesives pursue the goal of attaining high tensile shear strengths in metal bonds at room temperature. However, from temperatures of about 80-100.degree. C., these values drop sharply.
DE-A 36 10 758 describes, for the first time, pulverulent coating compositions based on EP hardeners which bring about the full curing of bisphenol A epoxy resins by polymerization of their epoxide groups and additionally by means of reaction of their hydroxyl functions. The one-component coatings thus produced are distinguished by very good coating properties and outstanding solvent resistance. The focus of application of these systems, however, is in powder coating, since the tensile shear strengths of the powders of DE-A 36 10 758 at elevated temperatures (DIN EN 1465) are poor, rendering them unsuitable for the bonding of metals.
DE 195 49 029 describes compounds that include cyclic amidine and urethane groups and are suitable as epoxy hardeners for the bonding of metals. The bonds have very good solvent resistance. Nevertheless, the cleavage of the urethane groups that is necessary for the urethane crosslinking reaction to proceed, necessitates relatively high curing temperatures.